This invention relates to improvements in angular velocity sensors, and in particular to an angular velocity sensor suitable for determining the angular position of the rotor in an electrical motor. It also relates to a method of determining the angular velocity of a rotor.
In this description the term “rotor” has been used to describe a device or element which is free to rotate about an axis. It may commonly comprise the rotor of an electric motor and indeed the invention primarily arose from work into monitoring the angular position of rotors in motors for subsequent use in a motor control strategy. However, it is to be understood that the term is not to be construed in the narrow sense and should instead be given a broader interpretation which includes—amongst other things—steering shafts for vehicle steering systems.
To measure the angular position of a rotor a wide variety of angular position sensors have been developed. The most common way of determining the angular velocity of a rotor is to provide a sensor, which determines the elapsed time taken for a fixed position on the rotor to make a complete revolution. The faster the rotor is spinning the shorter the elapsed time taken for the fixed point to pass the sensor and complete a revolution before passing the sensor again.
A problem with sensing the passing of a fixed point on a rotor is that a velocity value is only produced once for each complete revolution. If the rotor is turning relatively slowly the update time for the velocity value may be too long for accurate measurements to be obtained. To improve on the response time of the sensor it is therefore common to provide a number of different fixed points equally spaced around the rotor. For example, four points at 90 degrees spacing may be provided. In this case a velocity signal can be generated when the rotor has passed through a quarter of a revolution.
An example of a widely used angular velocity sensor which produces more than one output for each revolution of the rotor is the use of a Hall effect sensing device to detect the passing of the poles of the magnets of an electric motor.
The state of the Hall effect sensors changes whenever a magnet edge passes the sensing region of the elements. Thus, the passing of the rotor magnets defines the fixed measurement points used by the sensor. As more than one magnet is used then more than one fixed point is provided within each revolution.
A problem common to any system in which a number of fixed points are detected as a rotor rotates arises due to inaccuracies in the positioning of the fixed points. In a simple system the fixed points are arranged at equal angular spaced positions as illustrated in FIG. 2(a) of the accompanying drawings. Six magnets 21,22,23,24,25,26 are spaced at equal angles α around a rotor 20.
If the angular spacing is known then the velocity is readily determined by measuring the elapsed time between adjacent fixed points passing the sensor. However, if the fixed points are not accurately positioned the spacing between adjacent points will vary and so the velocity determined simply from the elapsed time will also seem to vary even if the rotor is rotating at a constant speed. This is illustrated in FIG. 2(b) of the accompanying drawings. The spacing α1 and α2 between two pairs of the magnets 21,22 and 22,23 is clearly unequal.
We have found that the exact angular position of the magnets of a practical rotor will be uncertain due to manufacturing tolerances and constraints associated with the attachment of the magnets to the rotor. This will cause the transitions between magnets to occur slightly before or after the expected angular transition point as the rotor is rotated. This misalignment is often random and cannot be predicted.
The problem of misalignment does not occur when only one fixed point is provided, as there will always be a complete revolution occurring between each sensing of the fixed point. For this reason, prior art velocity techniques have relied upon measurement over a complete revolution where high accuracy is needed or have simply tolerated the errors that occur due to misalignment.